1. The Field of the Invention
This invention relates to apparatus for treating liquids by reverse osmosis, and more particularly for releasing concentrated liquid from such apparatus.
2. The Prior Art
Reverse osmosis apparatus for treating liquids by reverse osmosis has generally included a pressure resistant container housing a semipermeable membrane or membrane cartridge. Feed liquid at elevated pressure is introduced into the pressure resistant container. Several types of valves and restrictors have been proposed and used for releasing concentrate (often referred to also as brine or blowdown) from the pressure resistant container at a controlled rate so that operating pressure is maintained therein. So-called back pressure control valves have been employed, which may be spring or gas pressure loaded and these are efficient but are complex and expensive. Various types of restrictors have been employed, mainly of the type which cause travel of the concentrate through an orifice or elongated, small diameter channel. An advantage of the long channel restrictor is that a tap may be made into the restrictor channel at a point intermediate its ends and which may be connected to the purified water storage container or system as described in U.S. Pat. Nos. 3,542,199 and 3,568,843. This provides pressure relief for the purified water system at a pressure intermediate the operating pressure and atmospheric pressure, and determined by the location of the tap along the length of the restrictor channel. It has been proposed to employ restricted channels formed by a spirally member abutting a flat plate or surface and an intermediate tap for storage tank pressure relief. Such restrictors are readily produced with a pressure relief tap, but are difficult to produce so that completely uniform channel dimensions are obtained. Even small variations in dimensions, as may be encountered in injection molded plastic parts, may result in channel dimension differences, and imperfect fit between mating pieces may also cause unreliable pressure control characteristics.
A small diameter plastic tube may be extruded to extremely accurate tolerances and can provide a desirable flow restrictor. However, an appreciable length of tubing is most often necessary for proper flow control, and storing several feet or more of tubing within a small membrane or cartridge-containing module presents some problems. Additionally, to form a tap by attaching a tube or fitting at a point intermediate its length is difficult and expensive, especially when it is considered that tubing for flow control for a small household reverse osmosis system, for example, may be of the order of a foot or several feet long, but only twenty-five thousandths of an inch internal diameter.